REBOL

Interesting programming language. http://www.rebol.com/

But not Open Source. Update (too late to matter?): (2012-12-13) Rebol Going Open Source

But what's interesting is that it's being used for Morpheus (v2) file-sharing, which means there could be a heck of a lot of Run-Time-s out there. Which could make this a promising Rich Client/IOS Platform. http://www.rebol.com/ios-intro.html

Loved by Chris Langreiter, used to build his Vanilla Wiki.